This invention relates to apparatus for controlling the working stroke of each hydraulic ram of a group of hydraulic rams, and to a sequence valve for use in such control apparatus. The invention is particularly concerned with apparatus for controlling the working strokes of the hydraulic props of a mine roof support unit.
Hydraulic roof support systems of the type used in longwall workings in underground mining operations usually comprise a plurality of self-advancing roof support units, each roof support unit typically has at least two, and in many cases four or more, hydraulic props which are connected to a hydraulic pressure line (which extends along the longwall working) by way of a common control valve. This control valve is usually a manually-operated and/or remote-controlled valve. When the valve is actuated, all the props of the associated roof support unit are simultaneously pressurized. In this case, the prop(s) that has (or have) the least resistance to extension move out before the prop(s) that has (or have) the greater resistance to extension. In the case of a roof support unit that has two props (or two pairs of props) arranged one behind the other in the direction of advance of the unit, and where the unit incorporates a roof cap which projects over a considerable distance towards the work face, the resistance to extension of the forward prop(s) is (or are) considerably greater than that of the rear prop(s). This is because of the different distribution of the weight of the roof cap on the props. Consequently, during the setting process, preferential outward thrust is applied to the rear prop(s). Indeed, in some circumstances, the front prop(s) is (or are) extended only when the rear prop(s) has (or have) already moved into contact with the roof. If, as is often the case in operations involving roof breakdown, the roof contains a cavity in the zone of the rear prop(s), the cap is forced into the cavity by the rear prop(s). In some circumstances, it is then not possible to contact the forward region of the sharply-inclined roof cap with the roof, and to clamp it against the roof by means of the front prop(s).
Where the props of a roof support unit have short extension lengths, and the unit has a goaf shield loaded with the weight of rubble, the resistance to extraction of the rear prop(s) may be, in some circumstances, greater than that of the front prop(s). In this case, the front prop(s) is (or are) preferentially extended during setting of the roof support unit. In this event, a roof cavity in the vicinity of the work face can result in difficulties similar to those occurring in the first-mentioned case.
As is well known, the hydraulic roof support units include, in addition to their props, a relatively large number of hydraulic rams, such as, for example, advance rams, cap-positioning rams, alignment rams, and rams for moving gap-sealing shields. All of these rams must be actuated in synchronism with the props, or in a timed sequence relative to the movement of the props.
The aim of the invention is to provide control apparatus which does not suffer from the disadvantages associated with known types of control apparatus. In particular, the invention aims to provide apparatus for controlling the working strokes of a group of hydraulic rams in such a manner that, despite the individual rams having different resistances to extension and different loads, their working strokes can be regulated to achieve synchronism or, if required, a timed sequence. Another aim of the invention is to provide an operationally-reliable sequence valve for such control apparatus, which valve offers the possibility of applying, as far as possible, the full pump pressure to the hydraulic rams of a group.